Hi, AFAIK when set to MAX they NEVER close no matter what.
The system has been bled (several times) including the filter unit that has its own little bleeder ( ).
Whenever the pump is running the filter unit makes a lot of noise as the water is forced through the baffle and past the magnetic separator tube before exiting out the other side.
Prior to the filter being installed this system was "silent" and had no overheat issues. So I think I will try to take it out of circuit as a next step. This will be a pretty binary test I think.
Any kind of filter will increase the pressure drop across the system and thereby the reduce flow hence the probs with overheating. If everything goes through the filter it doesn't matter what you do with the rads. I'm no plumberer but a bypass around the filter for normal use (why filter it all the time) might be an idea?